Understanding how crop diseases and climate change interact is vital

IN THE PAST 150 years, the concentration of carbon dioxide in the atmosphere has risen from 280 parts per million (ppm) to 410 ppm. For farmers this is mixed news. Any change in familiar weather patterns caused by the atmospheric warming this rise is bringing is bound to be disruptive. But more carbon dioxide means more fuel for photosynthesis and therefore enhanced growth—sometimes by as much as 40%. And for those in temperate zones, rising temperatures may bring milder weather and a longer growing season. (In the tropics the effects are not so likely to be benign.) What is not clear, though, and not much investigated, is how rising CO levels will affect the relation between crops and the diseases that affect them.History suggests that is an oversight. Devastating crop diseases do suddenly emerge from obscurity—often becoming epidemic far from their place of origin. In the 1840s, for example, a hitherto obscure fungus from Mexico devastated the Irish potato crop for several years, bringing about a famine that killed a million people. It would not be at all surprising if a changing climate led to conditions that caused similar epidemics.